The origins of life …

I have been reading Richard Fortey’s book “Survivors” (see “”They don’t do much, do they?””) which is a rare and precious thing in the world of natural history writing as he devotes two whole chapters to the algae.   The first of these describes his encounters with living stromatolites in Shark Bay, in Western Australia.   The point he is making as he writes is that he did not just make an extraordinarily long journey to get there (first, get to Perth, then travel a further 800 km north …) but that he is also making a similarly long journey back in time, as stromatolites are survivors from the Precambrian era and, indeed, may have played a vital role in creating the oxygen-rich atmosphere that we take for granted.

I have a polished specimen of a stromatolite, purchased from a fossil shop in Durham, which shows the characteristic fine laminations. Each of these represents a layer of Cyanobacteria (blue-green algae) filaments which have, in turn, trapped sediment particles.   The laminations are, in turn, formed into dome-like structures, reflecting the vertical growth of the filaments in search of sunlight.   This stromatolite is from the Ordovician era (I think), which dates it to between 443 and 485 million years ago.

stromatolite_argentina

A stromatolite from Argentina, possibly from the Ordovician era. The specimen is 10 cm long.

Not only were stromatolites extremely abundant in the Precambrian (approx. 4.6 billion to 540 million years ago) but the organisms from which they were formed appear to be very similar to Cyanobacteria that can still be found today.   They are extremely delicate structures that thrived, at least in part, because there were few other organisms in the Precambrian that could compete with them or graze them and, consequently, had shallow marine habitats to themselves for an extremely long time. During this period, they were busily photosynthesising away, taking carbon dioxide and water and converting it to simple sugars (which they needed to grow) and oxygen (which was, as far as the Cyanobacteria was concerned, a waste product). This oxygen was released as tiny bubbles (see “Ecological yin and yang…”) which, ever so slowly, accumulated in the atmosphere.   Maybe it is no surprise that the Precambrian, the era before fossils of multicellular organisms are common, lasts for about four fifths of the entire lifespan of the earth: it took this long for all those tiny bubbles to add up to enough oxygen to allow more complicated organisms to survive.

And what did those multicellular organisms feed upon?   That’s right: the Cyanobacteria had sown the seeds of their own destruction.   There is evidence not just of a gradual decline of stromatolites through the later Precambrian and into the Cambrian and Ordovician eras, but also of a resurgence of stromatolites after the mass extinction at the end of the Ordovician (which would have removed the multicellular grazers and left our tough little Cyanobacteria behind).   Stromatolites are found sporadically throughout the fossil record and in a small number of locations in the present day, but their heyday lies far in the past.

References

Bengston, S. (2002). The early worm catches the – what?   Pp. 289-317. In: The Fossil Record of Predation (edited by Kowalewski, M., and Kelley, P.H.). The Paleontological Society Papers 8, The Paleontological Society, Boulder, Colorado.

Fortey, R. (2011).   Survivors: the Animals and Plants that Time Has Left Behind.   Harper Press, London.

Sheehan, P.M. & Harris, M.T. (2004). Microbialite resurgence after the Late Ordovician extinction. Nature (London) 430: 75-78.

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Every (fifth) breath you take …

Ever on the hunt for a good ecological metaphor, I enjoyed David Mann’s suggestion that we should try missing out every fifth breath we take in order to appreciate the contribution that diatoms make to global productivity.   Approximately 20% of the oxygen that we breathe comes from diatoms, principally in the world’s oceans.   You can see the whole article here:  http://www.scotsman.com/news/little-word-about-the-mighty-diatom-1-3227674.

Paul Falkowski of Rutgers University, New Jersey makes an even greater claim for the impact of diatoms on humans: put simply, we would not be here without them.   His reasoning goes something like this: oxygen concentrations in the atmosphere have approximately doubled over the past 205 million years, a consequence, he argues, of the evolution of the diatoms and coccolithophorids. This increase in oxygen concentrations, in turn, facilitated the expansion and diversification of mammals, who depend upon the transfer of oxygen across the placenta.  Higher oxygen concentrations allows larger mammals to evolve and survive.  Ergo … no diatoms, no humans.

Reference

Falkowski, P.G., Katz, M.E., Milligan, A.J., Fennel, K., Cramer, B.S., Aubry, M.P., Berner, R.A., Novacek, M.J. & Zapol, W.M. (2005).  The rise of oxygen over the past 205 million years and the evolution of large placental mammals.   Science (New York) 309: 2202-2204.

Who do you think you are?

As well as the diatom growths, the bed of the River Browney was also covered with skeins of a green-coloured alga which, when I removed it, had a soft, felt-like texture.   This is Vaucheria, a very common constituent of enriched rivers in Britain.   Under the microscope, the filaments can be seen to be long tubes (think of sausage skins) within which there are numerous tiny green chloroplasts.   This is not the first bright green alga that I have written about in this blog but appearances, in the world of algae, can often be deceptive.

Browney_Vaucheria

We often see the evolutionary history of life on earth portrayed as a tree whose branches, representing each of the main groups of organisms, diverge again and again, culminating in “twigs” representing each species.   Following any “twig” back towards the “trunk” links to successively larger groupings of organisms.   So, for example, the species to which we belong, Homo sapiens, has no very close relatives, but is linked at the next level (family) to apes such as the chimpanzee.  Humans and chimps are, in turn, linked to the primates (order) such as gibbons and lemurs, which are part the mammals (class), including elephants and lions, which is part of the chordata (phylum) which links us to fish and reptiles.   Finally, the chordate belong to the Animalia (Kingdom) which links us to flies and slugs, and Animalia is part of the Domain Eukaryophyta, which links us to the rest of life except for bacteria.

We can use this analogy to express the relationship between any two organisms in terms of the number of steps along the tree before we find a common relative.  If we compared Vaucheria to Bulbochaete, which we met on 16 August, another bright green growth from the bed of a river, surprisingly we have to take eight steps (equivalent to comparing humans with plants!).   By contrast, the distance between Bulbochaete and Spirogyra is six steps (humans v fish) and between Vaucheria and Melosira (4 September) is a mere four steps (humans v gibbons).

The message is that the affinities amongst the algae are often not best discerned through seemingly obvious characteristics such as colour and shape but through biochemical composition, similarities in reproduction and the life cycle and in the structure of the DNA.  The other message is that the umbrella term “algae”, usually bit-players in any consideration of life on earth, embraces as much diversity as a typical zoo.   This is too easily forgotten, at least in part because they lack the televisual qualities that lend themselves to wildlife documentaries.   Unfortunately, in the process, we also often lose sight of the importance of algae in ecosystems.

Note

There is no universally agreed system of higher classification (see post of 11 August).  For this exercise I used the tree of life project (www.tolweb.org) as the basis for the classification of animals and Algaebase (www.algaebase.org) for the algae.